Currently commercialized organic light emitting diodes (OLED) panels have been prominent due to an excellent contrast ratio, a low degree of heat being generated therein, a thin and lightweight exterior and the like. The definition of the screen of the panel may be determined by increasing a difference in contrasts between dark portions and bright portions of the screen to allow the dark portions to have a greater degree of darkness and the bright portions to have a greater degree of brightness.
In general, in the case of liquid crystal displays (LCD) using liquid crystals, since a contrast ratio may be controlled by interposing two polarizing plates having polarizer elements arranged to be perpendicular to each other, between the liquid crystals, in a state in which a light emitting portion is constantly turned on, limitations in implementing a completely black screen have been present therein. However, since the OLED panel may itself be turned off in order to realize dark portions in a screen thereof, it may be considered to be closer to a black panel, as compared to a general LCD. However, this may be enabled in a case in which light introduced to the panel from the outside is effectively blocked. That is, external light may pass through peripheral portions of a polarizing plate to become linearly polarized light oriented in a single direction and then, the linearly polarized light may pass through a ¼ wavelength plate and be converted into a circular polarized light. Through such variations of light, in a case in which external light is introduced into the OLED panel, the light may be blocked from being emitted from the panel to thereby realize a black panel. Therefore, currently used OLED panels may necessarily require a ¼ wavelength plate in order to adjust visual characteristics of a screen thereof.
When the linearly polarized light passes through the ¼ wavelength plate, whether or not a perfectly circular polarization is implemented may be mainly determined depending on the ¼ wavelength plate. To this end, an in-plane retardation value of the ¼ wavelength plate may be designed to be about 140 nm at a reference wavelength of 550 nm in the visible light region. However, in order to provide perfectly circular polarization in the overall wavelengths of 400 nm to 750 nm in the visible light region, the in-plane retardation value may need to be about 100 nm at a wavelength of 400 nm and about 180 nm at a wavelength of 750 nm.
However, since a general polymer such as polycarbonate, a polycycloolefin or the like, commonly used for the ¼ wavelength plate in the related art, may have wavelength dispersion characteristics in which birefringence is greater in accordance with a reduction in wavelengths of light, that is, positive wavelength dispersion characteristics, it may be inappropriate to be used in the ¼ wavelength plate.
Therefore, as a method of controlling a wavelength in the overall visible light region, a method of stacking two or more birefringent films having different retardation wavelength dependences, at a certain angle, has been known. However, such a method may require a process of attaching a plurality of retardation films, a process of adjusting an angle at which the retardation film is attached, and the like, thereby leading to defects in productivity. In addition, since an overall thickness of the retardation film is increased, visible light transmittance may be degraded to cause darkness.
Recently, a method of controlling a wavelength in a broad band by using a single sheet of film, without the stacking described above, has been proposed. That is, a method of using a polycarbonate copolymer formed of a unit having positive refractive index anisotropy and a unit having negative refractive index anisotropy. However, since the polycarbonate copolymer contains a unit derived from bisphenol fluorine, it may have limitations such as a high melting temperature or susceptibility to gellification due to decomposition during melting processing. In addition, the polycarbonate copolymer may be defective in that a glass transition temperature (Tg) is relatively high, and a high temperature is required for a film stretching process, such that specific processing equipment different from that of the related art may be necessary.
In addition, a method of manufacturing a retardation film, using a polycarbonate copolymer containing fluorene rings and Isosorbide components, has been suggested. However, since the polycarbonate copolymer has low thermal stability, it may be inappropriate for use in a process such as melting processing or the like.
Therefore, the development of a resin composition used for manufacturing an optical film having reverse wavelength dispersion characteristics, while being easily manufactured may be urgently required.